A push to perform more basic studies has emerged in the HIV vaccine field given the often poor results in vaccine trails. As HIV is an immunosuppressive virus and many of its proteins alter cellular function, an important basic question that needs to be answered is what effect does using HIV proteins in a vaccine have on induced immune responses. We demonstrated that HIV Env, when delivered as a vaccine that presents cell-associated trimers, suppresses antigen-specific CD4+ T cell responses. We demonstrated that functional trimeric Env aberrantly signaled CD4+ T cells and blocked their expansion, which could lead to a deficiency in help for neutralizing antibody and CD8+ T cell responses. We propose to develop an Env immunogen that when presented as a functional cell surface trimer does not signal through CD4 thereby preserving CD4 help and retains epitopes capable of inducing broadly cross-reactive neutralizing antibodies. This Env should induce stronger CD8+ T cell responses and neutralizing antibodies because of better or altered exposure of neutralizing epitopes, which will be selected for, and increased CD4 help. We will also study whether Env immunogens that retain the ability to mediate entry and cell-cell fusion will present functional domains that are most relevant to neutralization and will provide additional advantages as immunogens. We will use a novel small animal model that will allow us to screen immunogens for both Env mediated immunosuppressive activities and T and B cell immune responses. In three specific aims, we will develop and test Env immunogens. Specific Aim 1: immunogen development. An Env that does not bind or signal CD4 and retains broad neutralization epitopes (CD4bs, CD4i, MPER, carbohydrate (2G12), V2-V3) will be developed. Specific Aim 2: analysis of immunogens for CD4+ T cell suppression and cross-reactive neutralizing antibody epitope expression. Immunogens will be sequentially screened for CD4 binding, CD4 signaling, and ability to suppress CD4+ T cell proliferation. They will then be analyzed for binding by, and, if possible, inhibition by broadly cross- reactive neutralizing monoclonal antibodies. Data from these analyses will be used in the further development of immunogens in Aim 1. The 20 best immunogens from Aim 2 will be studied in Aim 3. Specific Aim 3: immunization of human CD4 and CD4/CCR5 transgenic mice for analysis of effective CD4+ T cell help, development of cross-reactive neutralizing antibody, and CD8+ T cell responses. We will test the hypothesis that Env immunogens that do not suppress CD4 help will have broader neutralizing antibody responses and better CD8 responses with regard to both frequency of antigen-specific cells and multifunctionality of cells. This study will analyze the effect of removal of the immunosuppressive activity of HIV Env on vaccine responses with the goal of developing a better immunogen. The need for an HIV vaccine to control spread is recognized as extremely important. Given the failure of recent clinical trials, studies that focus on the basic biology of vaccine development are warranted. This grant will study and develop new HIV envelope immunogens for vaccine development.